This invention relates to a solenoid operated 3-way, 2-position directional control valve, and in particular such a valve for an antiskid regulation system comprising an armature which is controlled by a solenoid and is firmly connected to a valve spool movable in a bore with the valve housing such that the valve spool enables an outlet port to communicate with a fluid source or with a return-reservoir connector.
Such a 3-way, 2-position directional control valve is known from the U.S. Pat. No. 3,144,047. This valve, however, can only take two certain positions. When the magnet coil is de-energized, the valve spool is held in one of the end positions distant from the magnet by the force of a spring, with one of the flow passages being opened, while when the magnet coil is energized, the valve spool is shifted into the other end position in which the other flow passage is opened. These two positions are completely independent of the pressures at the different connectors or flow passages.
In many applications, in particular in antiskid regulation systems, it is necessary that a certain pressure be adjusted at the outlet port. From the German Patent DT-PS 1,203,623, an antiskid regulation system is known in which the effective brake pressure is regulated with a solenoid operated 3-way, 2-position directional control valve. The antiskid regulation system disclosed therein is to serve hereinafter as an example of the problems occurring when a pressure is regulated with such a valve. In the example of an antiskid regulation system, the pressure of a fluid source is delivered to the wheel brakes of an automotive vehicle via the 3-way, 2-position directional control valve. If it is ascertained by a controller that the brake pressure at the wheel brakes is too high, the magnet coil of the 3-way, 2-position directional control valve will be energized and the valve spool will be shifted in such a way that the outlet port will be connected with the return-reservoir connector and that the connector of the fluid source will be closed. In this manner, fluid flows from the wheel brakes to the return reservoir so that the pressure effective at the wheel brakes is reduced. If the 3-way, 2-position directional control valve is held in this position for a sufficiently long time, the pressure at the outlet port and, therefore, the effective brake pressure will be reduced to zero so that there will be no braking effect of the connected wheels.
However, in such an antiskid regulation system it is desired to bring the pressure at the outlet port of the 3-way, 2-position directional control valve and, therefore, the effective brake pressure to a certain value lying between the pressure of the fluid source and zero. But this is only possible with the known solenoid operated 3-way, 2-position directional control valve in that normally the valve spool is permanently switched from its one end position to its other end position so that the outlet port communicates either with the fluid source or with the return-reservoir connector for a very short time, respectively. In this process, fluid is fed to the slave unit or wheel brake cylinder for a very short time resulting in a rise of the pressure at the outlet port and in a consequent discharge to the return reservoir the pressure is reduced. In this process, the pressure varies permanently about the desired value.
However this above process results in a very high fluid consumption so that the fluid source has to provide fluid permanently although no more fluid would be required at the outlet port if the desired pressure value was reached. Moreover, the permanent pulsation of the effective pressure has disadvantageous effects on the slave units. In the example of the antiskid regulation system, the effective pressure pulsation leads to an intermittent braking which has an annoying effect on the vehicle occupants and can cause wrong reactions of the driver.